This invention relates to a method of preparing a polyvinyl foam composition, particularly to a method of preparing a polyvinyl foam composition that can render a low density, non flammable rigid foam.
It is known that polyvinyls resin, for example, polyvinyl chloride and its copolymers, have advantages in chemical resistance and flame resistance, and the foam products thereof are widely used for insulating material. However polyvinyl foams made in early days have disadvantages that they are liable to shrink and easily crack due to heat stress when they are employed at the extremely low temperature, such as, the temperatures of liquified nitrogen and liquified natural gas (LNG). Generally, polyvinyl rigid foams are produced, in a closed mold under high pressure, from a composition containing polyvinyl resin, filler, blowing agent, solvent and other additives. Improvements have appeared by developing formulation and processing technique thereof. It has been known that polyvinyl chloride resin is subject to heat degradation accompanied by emission of hydrogen chloride at the elevated processing temperature. Therefore it is desirable to reduce the resin content and increase the filler content to minimize the above defect as well as to improve the shrinkage resistance and flame resistance. In Japanese Pat. Nos. 46-2184, 52-26776, 53-7944, higher filler contents are used in the presence of a blowing agent, liquified butane, under specifically arranged condition, but from the compositions thereof, highly expanded or low density foams were not obtained. Likewise, in Japanese Pat. No. 56-129236, higher filler content is used in the presence of additional nitrile compound, such as acrylonitrile and the process is done in a usual way. It is also found that the foam produced therefrom does not render satisfactory properties of light weight and good mechanical strength.